Introduction to Volume 7
In this volume recent progress in synthetic coordination chemistry, which has led to the produc-tion of materials displaying nanoscopic structural motifs, is described. The availability of increas-ingly powerful structure determination methods such as area detection for single-crystal X-ray diffraction and high-energy electron microscopies has been a key aspect to the development of this area. It is now possible to determine the structures of very large clusters, aggregates of metal ions, and coordination polymers to atomic resolution on a routine basis. For example, the field of coordination polymers has been explosively developed thanks to the advances in X-ray diffraction methods. The results of such structure determinations are fed back to the precise design of the structures and properties of coordination materials. For example, unique properties of coordin-ation polymers (gas absorption, magnetism, etc.) have been explored through molecular-level designing.
The first eight chapters of this volume explore the emerging worlds of high nuclearity clusters, coordination polymers, and supramolecular systems. Naturally, some of these areas have points of overlap but it is convenient to consider the underlying structural motifs as defining the area of interest. Since the publication of the first edition of CCC (1987) these areas have become firmly established and the emerging importance of nanoscale structures has led to the development of synthetic strategies for producing materials based on coordination chemistry principles where the molecular entity builds up to a nanostructured material. The main aim of this volume is to illustrate this by considering the synthetic and structural aspects associated with this concept. In addition the aspects of the properties of such systems are discussed. These properties are often inexplicable in terms of simple molecular or macroscopic descriptions demanding considerable efforts in developing theoretical expressions to elucidate the observed behavior. Such unusual behavior points towards applications utilizing quantum effects and this aspect has been a major motivation for the huge synthetic efforts currently being applied to the area.
Active areas in coordination chemistry that are rapidly growing after CCC (1987) rely on the explosive development of nano science and technology in recent years. In contrast to the ‘‘top– down approach’’ from physical structures, the ‘‘bottom–up approach’’ from chemical components (i.e., molecules) has been showing remarkable potential for constructing well-defined, functional nanostructures. Coordination chemistry provides an ideal principle for the bottom–up design of molecules because metals and ligands naturally and spontaneously associate with each other through coordination interaction, giving rise to discrete and infinite structures in the nanoscopic region very efficiently. This approach produces not only nanosized structures but also nanoscopic functions, which is intrinsic to nanosized species due to the versatile properties latent in such transitions. This bottom–up approach to nanomolecules and materials is well documented in most of this volume.
Particularly noteworthy is the fact that the bottom–up approach has created new materials and functions which may open up commercial applications. For example, the gas-absorption property of nanoporous coordination materials, which are spontaneously formed from metals and rela-tively simple ligands in a very efficient fashion, has been explored only in recent years, and are becoming very promising candidates for hydrogen storage for fuel batteries.
In the first chapter the synthesis and structures of new heteropolyoxoanions and related systems are discussed. Such systems can enclose nanoscopic spaces and can be regarded as ‘‘nanoreactors’’. Clusters containing fragments of the lattices of semiconducting materials such as CdSe provide a vivid illustration of the transition from molecular-based to extended solid properties and show how the properties in the nanoscale region differ from those at each extreme. These are described in the second chapter. A third physical property for which a bounded system
the most rapidly expanding area and the major interest in this field is shifting from structure to function. Accordingly, gas adsorption properties of nanoporous coordination networks are well discussed.
Templating and self-assembly, which are two major synthetic strategies of supramolecular coordination compounds, are focused upon in Chapters 7 and 8. Both methods have shown powerful potentials for the construction of well-defined nanoarchitecture with interesting proper-ties. Although these methods were previously employed among organic chemists by using organic interactions (hydrogen bonding, van der Waals interactions, etc.), the coordination approach has recently been recognized to be the most efficient strategy for templating and self-assembly thanks to the variation of metal centers and their wide spectrum of coordination geometries. The dynamic properties of coordination assemblies are the current topic in this field, and switchable systems in which molecular motion and function can be controlled by the redox and photo activation of metal centers are focused upon.
In Chapter 9 two areas where single-crystal X-ray diffraction experiments cannot be used to explore the structures of the materials are reviewed. In effect, these are areas where coordination-based materials are processed to give new materials. Research into liquid crystals has burgeoned since CCC (1987) was published and the area of specific interest to coordination chemists, that of metallomesogens, has been developed in order to build in the advantages of incorporating metal centers into these phases. This is a rapidly expanding field which could lead to all sorts of ‘‘smart materials’’, some of which might combine the sorts of systems discussed in the earlier chapters of the volume with mesogenic properties. Chapter 10 discusses another route to processing coord-ination compounds using sol–gel processing. This is another area new to CCCII with the possibility of producing materials with quite unusual features, such as thin films and glasses, which have potential applications in a variety of fields.
Whilst we have tried to present new research areas where molecular-based compounds extend to the nanometer-length scale in their overall structures, we were unfortunately not able to include one aspect of relevance to this idea, that of Biomineralization. This field has enjoyed considerable interest since the availability of powerful electron microscopes made it possible to look at the intricate details of the beautiful macroscopic architectures found in the mineralized structures of a variety of creatures at the nanoscale level. It has become clear through this research that much of the ‘‘crystal engineering’’ which is required to create phase- and function-specific structures, often with amazing control over the precise shape of the resulting biomineral, must utilize coordination chemistry principles with the idea put forward that various ligating species become involved during mineral formation to act as templates or growth inhibitors.
Although the vast majority of biomineral structures are composed of calcium carbonates and phosphates or silicate-based materials and therefore outside the scope of what we define as Coordination Chemistry, there are some very important transition metal-based systems, especially the iron oxides and oxyhydroxides, where the biominerals can provide important insights into the coordination chemistry approaches utilized by biological systems. The specific case of the iron(III) oxyhydroxide mineral utilized by organisms to store iron in ferritins is discussed in detail in Chapter 8.7 of Volume 8 of this series. In ferritin, the iron oxyhydroxide is stored inside a hollow spherical cavity of 7–8 nm diameter surrounded by 24 (or 12 dimeric) protein subunits. In this chapter, the general principles in the operation of controlling iron hydrolysis to create iron biominerals are discussed with reference to the coordinating species which can be involved in directing the phase and function of the mineral. Ferritins are also particularly relevant to the research discussed in our volume, since they consist of encapsulated nanoscopic particles where the ‘‘ligands’’ are still clearly visible (the protein shell of the system).
Although as has been stated above, most biominerals are based on what is readily available to organisms for forming structures, calcium and silicate-based systems, there are some very impor-tant lessons to be learned by coordination chemists aiming towards ‘‘new materials’’. We need only think of the strength of rather light bones, which are some ten times stronger than ordinary concrete. When we consider that it is necessary to reinforce concrete with iron wires to achieve an
equivalent strength, and the disadvantages of this material in terms of weight, durability, and self-repair compared with our bones, we can appreciate that the composite material nature has come up with is far superior to anything we can currently create. The construction of calcium carbonate shells gives further insights into design principles we might wish to employ. As well as the different types of crystal growth to give different shapes, which can variously be described in terms of logarithmic growth, linear growth, concentric growth, and so on, there is also the creation of superstructures with careful layering of the mineral to reinforce a weak shear direction or else a change of phase on traveling from the inside of, say, an oyster shell, which is lined with mother-of-pearl (aragonite as nacre), to the tough outside made up of calcite.
In addition to such structural marvels, biomineral structures are used as sensors with some marine species displaying the structural motifs found in photonic crystals. As well as light sensing, calcium carbonate in the form of nanoparticles is used in the human ear as part of a gravity sensor and helps to keep us upright – it serves a similar purpose along the lateral line of fish. Perhaps most intriguing are the magnetic sensors, usually in the form of aligned and elongated nanocrystals of magnetite, found in the tissues of a variety of creatures including bacteria, bees, fish, and birds, which sense the Earth’s magnetic field and help these creatures to orientate themselves.
In Chapter 11, Molecular Electron Transfer, the broad and deep field of electron-transfer reactions of metal complexes is surveyed and analyzed. In Chapter 12, Electron Transfer From the Molecular to the Nanoscale, the new issues arising for electron-transfer processes on the nanoscale are addressed; this chapter is less a review than a ‘‘toolbox’’ for approaching and analyzing new situations. In Chapter 13, Magnetism From the Molecular to the Nanoscale, the mechanisms and consequences of magnetic coupling in zero- and one-dimensional systems com-prised of transition-metal complexes is surveyed. Related to the topics covered in this volume are a number addressed in other volumes. The techniques used to make the measurements are covered in Section I of Volume 2. Theoretical models, computational methods, and software are found in Volume 2, Sections II and III, while a number of the case studies presented in Section IV are pertinent to the articles in this chapter. Photochemical applications of metal complexes are considered in Volume 9, Chapters 11–16, 21 and 22.
In addition, subjects such as molecular photochemistry and photophysics and optical properties from the molecular to the nanoscale are closely related. Accordingly, a brief selection of lead-in references in these areas is provided. The organization and selection are strongly influenced by the interests of the author. Where possible review articles are cited rather than primary literature. At present the best consistent medium for review articles on inorganic photochemistry is Coordin-ation Chemistry Reviews.
1. Molecular Photochemistry and Photophysics: General References
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Roundhill, D. M. Photochemistry and Photophysics of Metal Complexes; Plenum: New York, 1994.
Horvath, O.; Stevenson, K. L. Charge Transfer Photochemistry of Coordination Compounds; VCH: New York, 1993.
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Kutal, C.; Adamson, A. W. In Comprehensive Coordination Chemistry; Wilkinson, G., Ed.; Pergamon: Oxford, UK, 1987, Vol. 1, pp 385–414.
Zuckerman, J. J., Ed. Inorganic Reactions and Methods; VCH: Deerfield Beach, FL, 1986; Vol. 15. Geoffroy, G. L.; Wrighton, M. S. Organometallic Photochemistry; Academic Press: New York. Adamson, A. W.; Fleischauer, P. D., Eds. Concepts of Inorganic Photochemistry; Wiley-Inter-science: New York, 1975.
Balzani, V.; Carassiti, V. Photochemistry of Coordination Compounds; Academic Press, New York, 1970.
Rate constants for quenching of the excited states of metal complexes are available through the Notre Dame Radiation Laboratory DataBasehttp://allen.rad.nd.edu/
Qu, P.; Meyer, G. J. Dye sensitization of electrodes. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: New York, 2001; Vol. 4, Part 2, pp 354–411.
Scandola, F.; Chiorbelli, C.; Indelli, M. T.; Rampi, M. A. Covalently linked systems containing metal complexes. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: New York, 2001; Vol. 3, Part 2, pp 337–408.
Striplin, D. R.; Crosby, G. A. Photophysical investigations of rhenium(I)Cl(CO)3 (phenanthro-line) complexes. Coord.Chem.Rev.2001, 211, 163–175.
Stufkens, D. J.; Vlcek, A. Ligand-dependent excited state behaviour of Re(I) and Ru(II) carbonyl–diimine complexes. Coord.Chem.Rev.1998, 177, 127–179.
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Schanze, K. S.; Macqueen, D. B.; Perkins, T. A.; Cabana, L. A. Studies of intramolecular electron and energy transfer using the fac-(diimine)Rei(CO)3 chromophore. Coord.Chem.Rev. 1993, 122, 63–89.
Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic Press: London, 1992.
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2.2 Other Metal Centers
Vlcek, A. Mechanistic roles of metal-to-ligand charge-transfer excited states in organometallic photochemistry. Coord.Chem.Rev.1998, 177, 219–256.
Vogler, A.; Kunkely, H. Photoreactivity of metal-to-ligand charge transfer excited states. Coord.Chem.Rev.1998, 177, 81–96.
Scaltrito, D. V.; Thompson, D. W.; O’Callaghan, J. A.; Meyer, G. J. MLCT excited states of cuprous bis-phenanthroline coordination compounds. Coord.Chem.Rev.2000, 208, 243–266.
Vogler, A; Kunkely, H. A new type of MLCT transition relevant to oxidative additions: d!* excitation. Coord.Chem Rev. 1998, 171, 399–406.
3. Ligand-to-metal Charge-transfer Excited States
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Stanislas, S.; Beauchamp, A. L.; Reber, C. The lowest-energy ligand-to-metal charge-transfer absorption band of trans-OsO2(malonate)(2) (2–). Inorg.Chem.2000, 39, 2152–2155.
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Yang, Y. S.; Hsu, W. Y.; Lee, H. F.; Huang, Y. C.; Yeh, C. S.; Hu, C. H. Experimental and theoretical studies of metal cation–pyridine complexes containing Cu and Ag. J.Phys.Chem.A 1999, 103, 11287–11292.
Kunkely, H.; Vogler, A. Photoreactivity of (HBpyrazolyl(3)) TiCl3 and (C5H5)TiCl3initiated by ligand-to-metal charge-transfer excitation. J.Photochem.Photobiol.A-Chem.1998, 119, 187–190.
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Carlos, R. M.; Frink, M. E.; Tfouni, E.; Ford, P. C. Photochemical and spectral properties of the sulfito rhodium(III) complexes trans-Rh(NH3)4(SO3)CN and Na(trans- Rh(NH3)4(SO3)2). Inorg.Chim.Acta 1992, 193, 159–165.
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4. Polyoxometallates/ Metal Oxo Complexes
Texier, I.; Delouis, J. F.; Delaire, J. A.; Gionnotti, C.; Plaza, P.; Martin, M. M. Dynamics of the first excited state of the decatungstate anion studied by subpicosecond laser spectroscopy. Chem. Phys.Lett.1999, 311, 139–145.
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Yamase, T.; Ohtaka, K. Photochemistry of polyoxovanadates. 1. Formation of the anion-encapsulated polyoxovanadate V15O36(Co3) (7-) and electron-spin polarization of alpha-hydroxyalkyl radicals in the presence of alcohols. J.Chem.Soc.– Dalton Trans.1994, 2599–2608.
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5. Metal-centered Excited States 5.1 Ligand Field Excited States
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5.2 Excited States of d10 and s2Systems
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5.3 Excited States of d2MN and MO Systems
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5.4 s2Metal Complexes
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5.5 fnMetal Complexes
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6. Ligand-centered Excited States
Wang, X. Y.; Del Guerzo, A.; Schmehl, R. H. Preferential solvation of an ILCT excited state in bis(terpyridine-phenylene-vinylene) Zn(II) complexes. Chem.Commun.2002, 2344–2345.
Vlcek, A. Highlights of the spectroscopy, photochemistry and electrochemistry of M(CO)4 -( -diimine) complexes, M¼ Cr, Mo, W. Coord.Chem.Rev.2002, 230, 225–242.
Del Guerzo, A.; Leroy, S.; Fages, F.; Schmehl, R. H. Photophysics of Re(I) and Ru(II) diimine complexes covalently linked to pyrene: contributions from intra-ligand charge transfer states. Inorg.Chem.2002, 41, 359–366.
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7. Outer-sphere Charge Transfer in Ion Pairs
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8. Metal–Metal Bonded Species
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9. Spectroscopy of Semiconductor Particles
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M Fujita Nagoya, Japan July 2003 A Powell Karlsruhe, Germany July 2003 C Creutz Upton, USA May 2003
From Biology to Nanotechnology
Second Edition Edited by
J.A. McCleverty, University of Bristol, UK
T.J. Meyer, Los Alamos National Laboratory, Los Alamos, USA
Description
This is the sequel of what has become a classic in the field, Comprehensive Coordination Chemistry. The first edition, CCC-I, appeared in 1987 under the editorship of Sir Geoffrey Wilkinson (Editor-in-Chief), Robert D. Gillard and Jon A. McCleverty (Executive Editors). It was intended to give a contemporary overview of the field, providing both a convenient first source of information and a vehicle to stimulate further advances in the field. The second edition, CCC-II, builds on the first and will survey developments since 1980 authoritatively and critically with a greater emphasis on current trends in biology, materials science and other areas of contemporary scientific interest. Since the 1980s, an astonishing growth and specialisation of knowledge within coordination chemistry, including the rapid development of interdisciplinary fields has made it
impossible to provide a totally comprehensive review. CCC-II provides its readers with reliable and informative background information in particular areas based on key primary and secondary references. It gives a clear overview of the state-of-the-art research findings in those areas that the International Advisory Board, the Volume Editors, and the Editors-in-Chief believed to be especially important to the field. CCC-II will provide researchers at all levels of sophistication, from academia, industry and national labs, with an unparalleled depth of coverage.
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Volume 9: Applications of Coordination Chemistry Volume 10: Cumulative Subject Index
Volume 7:
From the Molecular to the Nanoscale:
Synthesis, Structure, and Properties
Edited by
M. Fujita, A. Powell, C. Creutz
C o n t e n t s H i g h n u c l e a r i t y c l u s t e r s : I s o a n d H e t e r o p o l y o x o a n i o n s a n d r e l a t i v e s ( L . C r o n i n ) H i g h n u c l e a r i t y c l u s t e r s : c l u s t e r s o n t h e t r a n s i t i o n f r o m s e m i c o n d u c t i n g t o m e t a l l i c ( J . F C o r r i g a n , M . d e G r o o t ) H i g h n u c l e a r i t y c l u s t e r s : c l u s t e r s a n d a g g r e g a t e s w i t h p a r a m a g n e t i c c e n t r e s : O x y g e n a n d N i t r o g e n b r i d g e d s y s t e m s ( R . E . P . W i n p e n n y ) H i g h n u c l e a r i t y c l u s t e r s : c l u s t e r s a n d a g g r e g a t e s w i t h p a r a m a g n e t i c c e n t r e s : C y a n o a n d O x a l a t o b r i d g e d s y s t e m s ( S . D e c u r t i n s , M . P i l k i n g t o n ) C o o r d i n a t i o n p o l y m e r s : i n f i n i t e s y s t e m s ( S u s a m u K i t a g a w a ) C o o r d i n a t i o n p o l y m e r s : d i s c r e t e s y s t e m s ( E . C o n s t a b l e ) S u p r a m o l e c u l a r s y s t e m s : t e m p l a t i n g ( J - P . C o l l i n e t a l . ) S u p r a m o l e c u l a r s y s t e m s : s e l f - a s s e m b l y ( K . N . R a y m o n d ) M e t a l l o m e s o g e n s ( D . W . B r u c e e t a l . ) S o l - g e l p r o c e s s i n g o f m e t a l c o m p o u n d s ( U . S c h u b e r t ) M o l e c u l a r e l e c t r o n t r a n s f e r ( J . F . E n d i c o t t ) E l e c t r o n t r a n s f e r f r o m t h e m o l e c u l a r t o t h e n a n o s c a l e ( C . C r e u t z e t a l . ) M a g n e t i s m f r o m t h e m o l e c u l a r t o t h e n a n o s c a l e ( D . G a t t e s c h i e t a l . )
L. CRONIN
University of Glasgow, UK
7.1.1 INTRODUCTION 2
7.1.1.1 Scope 2
7.1.1.2 Fundamental Units and Building Blocks 2 7.1.1.3 Basic Principles in Polyoxometalate Cluster Synthesis 3
7.1.2 VANADATES 4
7.1.2.1 {V12} Clusters 5
7.1.2.2 {V14} and {V15} Clusters 8
7.1.2.3 {V18}, { V22}, {V34} Clusters—Clusters Shaped by Encapsulated Templates 10
7.1.3 TUNGSTATES 15
7.1.3.1 Clusters Incorporating Monovacant Lacunary Fragments 18
7.1.3.1.1 {XW11}2 18
7.1.3.1.2 {XW11}2{Mo3S4}2 18
7.1.3.1.3 {XW11}3 19
7.1.3.2 Clusters Incorporating Different Types of Trivacant Lacunary Fragments 19
7.1.3.2.1 {X2W21} 19 7.1.3.2.2 {M9P5W27} 21 7.1.3.2.3 {XW9}1:{Eu3SbW24} 21 7.1.3.2.4 {XW9}2:{X2W21}/{X2W22} 22 7.1.3.2.5 {XW9}3 22 7.1.3.2.6 {XW9}4 24 7.1.3.2.7 {XW9}11 24
7.1.3.3 Clusters Incorporating Hexavacant Lacunary Fragments 24
7.1.3.3.1 {P8W48} 24
7.1.3.3.2 {P5W30} 24
7.1.4 MOLYBDATES 26
7.1.4.1 From Keggin Ions to {Mo37} Clusters 27
7.1.4.2 From {Mo36} to {Mo57} Clusters—Two and Three Fragment Clusters Based on {Mo17} Units 27
7.1.4.3 {Mo154} Big Wheel Clusters 28
7.1.4.3.1 Construction of{Mo154}-type clusters 29
7.1.4.3.2 Determination ofthe molecular formula of{Mo154}-type clusters 30
7.1.4.4 Reactions of the {Mo154}-type Wheels 30
7.1.4.4.1 Formation ofstructural defects 30
7.1.4.4.2 Linking ofwheels to chains and layers 32
7.1.4.4.3 Formation ofhost guest systems 32
7.1.4.4.4 Structural modifications of the big wheel clusters 36
7.1.4.5 {Mo176} Wheel and Derivatives 37
7.1.4.5.1 Comparison between the{Mo154}and{Mo176}big wheel clusters 37
7.1.4.5.2 Nucleation processes within a cluster cavity—from a{Mo176}to a{Mo248}cluster 38
7.1.4.5.3 Surface ligand exchange on the big wheel clusters 39 7.1.4.6 Synthesis of Wheels with Electrophiles 40 7.1.4.6.1 Synthesis ofthe big wheel-type clusters with PrIIIsalts 40 7.1.4.6.2 Synthesis ofthe big wheel clusters with EuIIIsalts 40
7.1.4.7 {Mo132} Big Ball Keplerate Clusters 42
7.1.4.7.1 Building block scheme for the Keplerate clusters 43 7.1.4.7.2 Construction ofspherical species with icosahedral symmetry 44
7.1.4.7.3 Changing the bridging ligands in the Keplerate clusters 44 7.1.4.7.4 Structural derivatives: removing the lid ofthe Keplerate 45 7.1.4.7.5 From{Mo132}to{Mo72M30}spherical clusters (M¼ Fe, Mo) 45
7.1.4.7.6 Formation ofmolecular barrels{Mo75V20} 46
7.1.4.7.7 Formation ofsolid-state structures with{Mo72Fe30} 47
7.1.4.7.8 Molecular hostages and networks ofmolecular hostages 48 7.1.4.8 {Mo368} Clusters: a Hybrid Between Wheel- and Ball-shaped clusters 49
7.1.4.9 Building Block Principles 51
7.1.5 OUTLOOK 52
7.1.6 REFERENCES 53
7.1.1 INTRODUCTION
Since the early 1980s the field of polyoxometalate chemistry has undergone a revolution. This has been characterized by the synthesis of ultra-large clusters that have nuclearities as high as 368 metal atoms in a single molecular cluster.1 Of course, such discoveries have only been possible
thanks to the advances of the instrumentation used to collect the diffraction data coupled with the advent of cheap and powerful computing power for structure solution and refinement. Much of the interest in these molecules has arisen because such clusters represent a paradigm in the discovery of systems that can be encouraged to grow from the molecular to the nanoscale. Polyoxometalates have also generated interest in areas as diverse as catalysis,2–13 magnetism,14–23 synthesis of molecular devices,24 synthesis of new materials,25–51 and have even found potential application as anti-viral agents.52–55
7.1.1.1 Scope
In this article the field of polyoxometalate chemistry will be reviewed and discussed as it has progressed from the 1980s to its position at the start of the new millennium. In embarking on this journey special attention will be given to the synthesis, structure, and properties of discrete polyoxometalate clusters with a nuclearity that is greater than 12 metal atoms. In nearly all cases the frameworks of these clusters are based upon V, Mo, and W. There is a rich chemistry with iso and heteropolyanions with nuclearities 12 and below (see also Chapters 4.10 and 4.11), but these will not be treated in this chapter unless they are used as fragments in the construction of larger clusters or have interesting physical properties.56,57
7.1.1.2 Fundamental Units and Building Blocks
Polyoxometalate cluster anions are comprised of aggregates of metal–oxygen units where the metal can be best visualized as adopting the center of a polyhedron and the oxygen ligands defining the vertices of this polyhedron. Therefore, the overall structures of the cluster can be represented by a set of polyhedra that have corner- or edge-sharing modes (face sharing is also possible but rarely seen), seeFigure 1for examples of corner- and edge-sharing polyhedra.
It is not surprising therefore that there are, at least theoretically, a bewildering number of structurally distinct clusters available for a given nuclearity. However, it will become evident that it is extremely useful, at least conceptually, to regard these metal-centered polyhedra and aggre-gates of these {MOx} polyhedra as structural building blocks that can be used to help both understand and perhaps even manipulate the synthesis of cluster. The structures can then be considered to form via a self-assembly process involving the linking or aggregation of these polyhedra.58,59 However, although such concepts will be widely considered here, care must be taken to distinguish between a structurally repeating building block and an experimentally available building block that can be proved to be present and incorporated during the construc-tion of a given cluster.60
7.1.1.3 Basic Principles in Polyoxometalate Cluster Synthesis
Before outlining the general approach to the synthesis of polyoxometalate clusters it is informative to consider the most useful synthetic results thus far discovered for derivatization and functionalization of fragments leading to a huge variety of structures. These are given below:
The potential of the system to generate a versatile library of linkable units.
The ability to generate groups (intermediates) with high free enthalpy to drive polymeriza-tion or growth processes, e.g., by formapolymeriza-tion of H2O.
The possibility for structural change in the building units or blocks. The ability to include hetero-metallic centers in the fragments.
The possibility to form larger groups that can be linked in different ways. The ability to control the structure-forming processes using templates.
The ability to generate structural defects in reaction intermediates (e.g., leading to lacunary structures) for example by removing building blocks from (large) intermediates due to the presence of appropriate reactants.
The ability to localize and delocalize electrons in different ways in order to gain versatility. The ability to control and vary the charge of building parts (e.g., by protonation, electron transfer reactions, or substitution) and to limit growth by the presence of appropriate terminal ligands.
The possibility of generating fragments with energetically low-lying unoccupied molecular orbitals.
The ability to selectively derivatize both the outside and inside of clusters with sizable cavities. Generally, the approaches used to produce high nuclearity polyoxometalate-based clusters are extremely simple, consisting of acidifying an aqueous solution containing the relevant metal oxide anions (molybdate, tungstate, and vanadate). In the case of the acidification of the metal oxide-containing solution (seeFigure 2) for example, the acidification of a solution of sodium molybdate gives rise to fragments, which increase in nuclearity as the pH of the solution decreases (see Section 7.1.4).56,57These isopolyanions have been extremely well investigated in the case of molybdenum, vanadium, and tungsten. However the tungsten cases are limited due to the time required for the system to equilibrate, which is of the order of weeks.56 Another class of cluster can be synthesized when hetero atoms are introduced, heteropolyanions (see Section 7.1.3) and these are extremely versatile. Indeed, heteroanions based on tungsten have been used in the assembly of extremely large clusters (see Section 7.1.3.2.7).61 In the case of
molybdenum the acidification of solutions of molybdate followed by its subsequent reduction yields new classes of clusters with interesting topologies and very large nuclearities (see Section 7.1.4).62,63
Figure 1 A representation of corner- and edge-sharing polyhedra found in polyoxometalate clusters. The metal ions at the center of the open polyhedra are shown by the black spheres and the oxygen ligands at the vertexes of the polyhedra are shown by smaller black circles. The top image shows exclusively a
The synthetic variables of greatest importance in synthesizing such clusters may be outlined as follows:
concentration/type of metal oxide anion; pH and type of acid;
type and concentration of electrolyte; heteroatom concentration;
possibility to introduce additional ligands (reducing ligands); reducing agent (in the case of the Mo systems);
temperature; and solvent.
Often such syntheses are done in a single pot and this can mask the extraordinary complexity of the assembly event(s) leading to the high nuclearity cluster. Specific reaction variables and considerations will be discussed at the relevant points throughout this chapter.
7.1.2 VANADATES
The vanadates are structurally very flexible and as such can be based on a large number of different types of polyhedra {VOx} where x¼ 4, 5, 6 whereby the pyramidal O¼VO4polyhedra show a tendency to form cluster shells or cages which have topological similarities to the full-erenes and comprise aspects that are structurally analogous to the layers of V2O5.64,65The bulk of the polyoxovanadates reported so far possess a variable number of vanadium ions bridged by 2-, 3-oxo, and -arseniato groups to yield complex structures ranging from approximate spherical to elliptical geometries.66,67 The geometry around the vanadium ions can be square pyramidal, octahedral, or tetrahedral. In the tetrahedral case the ion is almost always vanadium(V), while in the square pyramidal/octahedral geometries the metal ion can either be in theþ4 or þ5 oxidation state. The resulting structures range from quite compact forms, for example, in the case of [V10O28]6to open ribbon, basket, shell, and cage-like host systems,64suitable for the uptake of neutral68,69 and ionic guests.70–73 In addition, two-dimensional layered materials,74,75 as well as
three-dimensional host structures,76 have been described in recent years. Interestingly, simple vanadates have even been found useful to replace insulin in some mammals.77–79
The identification of the oxidation state of the square pyramidal vanadium ions is not always easy, especially when extensive electron delocalization is present. However valence bond summa-tions can greatly aid the assignment in those cases where sufficiently high quality structural data have been obtained. Such assignments can be further checked by EPR and magnetic investiga-tions. Indeed, one of the most exciting aspects of polyoxovanadate chemistry is the prospect of synthesizing topologically80–82 interesting clusters that can behave as nanoscale magnets.19,83–88 Such clusters are synthesized in aqueous solution with the appropriate precursor, anion templates and, in the case of the mixed valence species, reducing agents. However vanadates have also been synthesized under hydrothermal conditions,89and even in vanadium oxide sol–gel systems.30,90
Polyoxometalates Mononuclear [VO4]3– [MoO4]2– [WO4]2– H+ Oxides V2O5 MoO3 WO3
Figure 2 Polyoxometalates are formed in experimental conditions that allow linking of polyhedra. Discrete structures are formed as long as the system is not driven all the way to the oxide. One such example, in this
valence vanadium cluster with both localized and delocalized vanadium centers (Figure 3). Com-pound (1) is formally built up by nine VO6octahedra, three VO4tetrahedra, and four AsO4 tetra-hedra, one of the latter being a central AsO43group. The terminal O atoms of each of the peripheral AsO4groups are protonated and a potassium ion crowns the fragment. The number of vanadium(IV) centers, expected to be four, was confirmed by Barra et al. by manganometric titration.93
The identification of the vanadium(IV) centers in the structure is not a trivial endeavor. Bond valence sum (BVS) investigations94 suggest that V10, V11, and V12 are localized vanadium(IV)
centers, however the fourth vanadium(IV) ion is delocalized over the positions V1, V2, and V3, i.e., a {V3þ1} cluster, see Figure 3. The oxovanadium ions V10, V11, and V12 are connected by long O–As–O bridges and the delocalized vanadium(IV) spread on positions V1, V2, and V3 are connected by 2-oxo bridges. The connection between the localized and delocalized vanadium(IV) ions are long and involve more atoms, so their interaction is negligible. The {V3þ1} electronic structure was also confirmed by magnetic measurements giving a room-temperature effective magnetic moment of 3.17B, which corresponds to four unpaired electrons. The magnetic moment decreases smoothly with decreasing temperature giving a small plateau at 2.36B in the range 10–20 K. This was modeled by including an exchange coupling constant, J, for the localized and J0 for the localized–delocalized interaction. The best-fit values were reported as being J¼ 63 cm1 and J0¼ 1.0 cm1. It would appear that this case provides useful information for the analysis of more complex systems, namely those in which there is ambiguity when judging the extent of delocalization vs. localization using the BVS approach. In addition the data indicate that the delocalization is extremely fast and thus one averaged coupling constant can be used.87,92 Synthesis of the isostructural clusters [V12As8O40(KCO2)]n (when n¼ 3 (2a) the cations are 2[HNEt3]þ and 1[HNH2Me]þ and when n¼ 5 (2b) the cations are five sodium ions) gave an opportunity to compare two isostructural clusters that have different ratios of VIV/V ions in the cluster framework, seeFigure 4. (2a) contains six noninteracting and (2b) eight antiferromagnet-ically coupled VIV(d1) centers.
Both cluster anions have D4h symmetry and consist of 12 distorted tetragonal VO5 pyramids and four As2O5 groups, which together link to form a hollow cavity that encapsulates a
V1 V2 V3 V4 V5 V6 V7 V9 V10 V11 V12 V8
Figure 3 A representation of the crystal structure of the {V12} cluster (1). The vanadium ions are shown as
black spheres, the arsenate ions by dark gray spheres and the potassium ion by the large light gray sphere. The small white spheres are oxygen atoms and the smaller white spheres are hydrogen atoms.
disordered formate ion.95,96 The 12 VO
5pyramids can be divided into two types that differ with respect to their position relative to the As2O5groups. The first consists of four pyramids that are bridged through edges by the As2O5groups forming the middle section of the anion where four VIV centers are trapped, see Figure 4, while two VIV centers in (2a) and four in (2b) are delocalized over eight sites, the remaining ions being formally VV ions, i.e., {V4þ2} and {V4þ4}, respectively. Although the magnetic analysis is quite complex it has been shown that the magnetic behavior correlates with the geometry and the topology of the cluster.96 The four localized vanadium(IV) ions are bridged by -O–As–O groups, while the delocalized sites are bridged by -O and -O–As–O groups, and the mixed localization sites are bridged by either double 2-OAs or single -O–As–O groups, see Table 1. The room-temperature effective magnetic moment of {V4þ2} is 4.05B and the {V4þ4} is 2.97B, indicating that in both cases there are many electrons with antiparallel spins. Overall, magnetic properties of {V4þ4} can be explained by assuming that the two vanadium(IV) ions in the delocalized sites are strongly antiferromagnetically coupled, so that the observed effective magnetic moment can be attributed to the four localized vanadium(IV) ions. Using this model the temperature dependence of the effective magnetic moment can be fitted with J¼ 10 cm1. The magnetic properties of the {V
4þ2} are more problematic as the data cannot be fitted with only antiferromagnetic coupling constants. However, if one constant is assumed to be ferromagnetic then a good fit is obtained, but the pathway that gives rise to this is difficult to assign.
Figure 4 Structure of the cluster (2a) and (2b) (left-hand side (LHS)¼ side view; right-hand side (RHS)¼ top view) with an encapsulated disordered formate ion (center of the RHS view). The trapped VIVcenters are shown by the arrows in the RHS view. The vanadium ions are shown as black spheres, the
arsenate ions by dark gray spheres and the oxygen atoms as white spheres.
Table 1 Exchange pathways and coupling constants in some vanadates—see reference87 for a more
advanced and complete discussion.
Cluster Atom 1 Atom 2 Bridge 1 Bridge 2 Distance Coupl Value {V15} (7) V1 V2 3-O 3-O 2.87 J 556 V1 V20 3-O 2-OAs 3.05 J0 104 V1 V3 3-O 2-OAs 3.02 J1 104 V2 V20 3-O 3.68 J00 208 V2 V30 3-O 3.73 J2 208 {V14} (8) V1 V2 2-OAs 2-OAs 3.06 J1 19 V3 V4 3-O 2-OAs 3.01 J2 124 V2 V4 3-O 3-O 284 J 507 V2 V3 3-O 3.60 J3 55 {V3þ1} (1) V10 V11 -OAsO 5.70 J 63
{V4þ2} (2a) V10 V11 -OAsO -OAsO 5.25 J 10
{V4þ4} (2b) V10 V8 2-OAsO 2-OAs 3.16
V10 V9 2-OAsO 5.28 J0 12
The anionic cluster83,97[H6V12O30F2]6(3) contains 10 VIVand two localized VVcenters and as such, this compound offers another test for the validity of valence bond summations, which suggest all the charges are trapped. Standard BVS calculations clearly indicate that the localized VV centers are those shown as V3 and V3a inFigure 5.
It is also possible to synthesize somewhat more open clusters. For example Klemperer et al. synthesized a topologically interesting vanadate, a [V12O32]4basket64,68 (4) which comprises 12 VV ions. Interestingly the basket holds an acetonitrile molecule, seeFigure 6.
This result was extended with the inclusion of C6H5CN in the molecular bowl (5), seeFigure 7.98 This result offers the possibility that vanadium oxide bowls could be used as molecular containers and may help capture and stabilize interesting molecules.
Indeed this approach was extended by Ozeki and Yagasaki99 in 2000 when they managed to
crystallize a {V12} bowl (6) analogous to those reported before, but this time encapsulating a NO anion, seeFigure 8.
This is the first example of the NOanion trapped in the solid phase and it is notable that the NOanion appears to rest deeper in the cavity than any of the previous guest molecules. This is of interest as an example of an anionic guest being isolated in an anionic host, but is by no means without precedent (seeSection 7.1.4.4.3).
V3a
F1
V3 F2
Figure 5 Structure of the cluster anion [H6V12O30F2]6. The vanadium ions are shown as black spheres, the
fluoride atoms by the gray spheres. The small white spheres are oxygen atoms and the smaller white spheres are hydrogen atoms.
Figure 6 Representation of the vanadate basket cluster, [V12O32]4(LHS¼ top view; RHS ¼ side view). The
acetonitrile solvent molecule can be seen in the center of the cavity. The vanadium ions are shown as black spheres and the white spheres are oxygen atoms.
7.1.2.2 {V14} and {V15} Clusters
One of the most interesting aspects of cluster synthesis is the possibility of engineering, by accident or design,100clusters with large but finite numbers of spins, which are coupled to each other. In this respect the cluster anion [V15As6O42(H2O)]6(7) comprising 15 VIVions,87,101offers interesting possibilities.
The overall structure of (7) is shown in Figure 9, and the cluster has crystallographically imposed D3 symmetry. It consists of 15 distorted tetragonal VO5 pyramids and six trigonal AsO3 pyramids and it encapsulates a water molecule at the center of the quasi-spherical cluster sheath. The 15 VO5pyramids are linked to one another through vertices. Two AsO3groups are joined to each other via an oxygen bridge forming a handle-like As2O5moiety.
Figure 7 A representation of the vanadate basket cluster, [V12O32]4including a C6H5CN molecule. The
vanadium ions are shown as the large black spheres and the white spheres are oxygen atoms. The smaller black spheres and the gray sphere indicated the C6H5CN molecule.
Figure 8 A representation of the crystal structure of the NOanion in a vanadate-based molecular bowl. The vanadium ions are shown as the large black spheres and the white spheres are oxygen atoms. The NO
The {V15} cluster (7) has a room-temperature effective magnetic moment of 4.0B, indicating strong antiferromagnetic coupling compared with the value for 15 uncoupled vanadium(IV) ions, which is 6.7B. The effective magnetic moment decreases slowly on decreasing temperature and in the region of 100–20 K it tends to 2.8B. Below 20 K eff decreases again reaching 2.0B at 1.8 K. It would appear that the observation that the effective magnetic moment is essentially constant over a large range of temperatures is an indication that the strong antiferromagnetic coupling leaves at least three spins uncoupled at high temperature, i.e., a smaller antiferromag-netic exchange interaction couples the three spins together at low temperature, see Table 1 for details of bridging and coupling constants. Detailed analysis has shown this cluster to possess a unique multilayer magnetic structure.102,103Briefly, (7) can be considered as a small model of a
multilayer structure with two external antiferromagnetic layers sandwiching an internal triangular planar layer, as schematically shown inFigure 10.
The cluster anion [V14As8O42(SO3)]6(8), which is shown inFigure 11, is also composed exclusively of VIVions. Of these, eight, which are connected by
3-O and 3-OAs groups, define an octagon, and then two sets of three VIVions connect diametrically opposed centers on the octagons. The room-temperature effective magnetic moment is 4.45 B, which is also smaller than expected for 14 uncoupled spins (6.48 B) clearly indicating the presence of antiferromagnetic coupling.102,103
Figure 9 A representation of the structure of (7) from the top (left) and the side (right) view respectively. The vanadium ions are shown as black spheres, the arsenate ions by dark gray spheres and the oxygen atoms
as white spheres.
In studies by Yamase et al.104 a {V15} cluster encapsulating a CO32 was synthesized and characterized, seeFigure 12. The [V15O36(CO3)]7anion (9) was synthesized by the photolysis of solutions of [V4O12]4 at pH¼ 9 adjusted by K2CO3. The resulting anion is a nearly spherical {V15O36} cluster shell encapsulating a CO32anion and formally contains eight VIVand seven VV centers. The structure of this cluster sheath is virtually identical to a {V15} cluster (9(a)) synthe-sized by Mu¨ller in 1987 of the formula [V15O36Cl]6.105
7.1.2.3 {V18}, { V22}, {V34} Clusters—Clusters Shaped by Encapsulated Templates
It would appear that under certain reaction conditions, vanadate cluster shells can be generated by linking fragments that depend to a large extent on the size, shape, and charge of a template (in most cases the templates are anions) incorporated as a guest in the final structure. The cluster
Figure 11 A representation of the structure of the {V14} cluster (8). The left view shows the central belt of
vanadium ions and the caps above and below the belt. The right view shows that the central belt comprises eight vanadium ions and the caps of three vanadium ions above and below the central belt. The vanadium ions are
shown as black spheres, the arsenate ions by dark gray spheres and the oxygen atoms as white spheres.
Figure 12 A representation of the structure of the {V15} (9) shell encapsulating a carbonate dianion. The
vanadium ions are shown as black spheres and the oxygen atoms as white spheres. The carbon atom of the carbonate anion is shown as a gray sphere.
interactions appears to facilitate very subtle sculpting of the resulting cluster cage. For example, it is possible to synthesize structurally equivalent {V18} cluster cages but with differing electron populations and guests encapsulated within the host.
V O O O O O X V O O O O O V O O O V O O O X n Scheme 1
It has been shown by Mu¨ller et al. that the {V18O42} shell can exist in two different structural types.106The 24
3oxygen atoms form either the edges of a distorted rhombicuboctahedron or a pseudorhombicuboctahedron (the ‘‘14th’’ Archimedian solid), see Figure 13.80,92 The latter poly-hedron can be generated by a 45rotation of one-half of the rhombicuboctahedron around one of
its S4 axes. Clusters corresponding to the rhombicuboctahedron can be regarded as being an enlarged Keggin ion, in which all the planes of the rhombicuboctahedron are spanned by 24 oxygen atoms and are capped by the {VO} units.
For example, the anion, [H7VIV16VV2O42(VO4)]6(10) adopts Td symmetry due to the highly charged, tetrahedral [VO4]3‘‘template’’ which seems to ‘‘force’’ the outer cluster shell to adopt the same symmetry, seeFigure 14. This cluster is different from the other {V18} clusters reported as the {VO4} unit is actually bonded to the cluster shell, whereas in the other clusters guest molecules are merely included in the cluster as a nonbonded fragment.
In the case of the other guests (Table 2) such as H2O, Cl, Br, I, SH, NO2, HCO2 the cluster adopts the D4dsymmetry, seeFigure 15.
Although only two structural types have been identified, within this structural classification there appear to be three types of redox states: (i) VIV18O42 (compounds (11a)—(11d)); (ii) VIV
16VV2O42 (compound (10)); and (iii) VIV10VV8O42(compound (12)) seeTable 2.
Type (i) clusters are fully reduced anions with 18VIV centers and encapsulate either neutral or anionic guests; the nature of the guest is responsible for any structural variation. Compounds
Figure 13 A schematic of the two types of polyhedron formed by the {V18} clusters. The Td
(11a–c) are synthesized under anaerobic conditions at high pH values (ca. 14) from aqueous vanadates solutions and under these conditions only water molecules are enclosed within the cluster shell. The incorporation of anions in the fully reduced shell is facilitated by synthesizing the clusters at a lower pH and (ca. 10) by addition of the correct anion. Type (ii) clusters are mixed valence anions (type III according to the classification of Robin and Day)107with encap-sulated anions. These compounds are synthesized under an inert atmosphere and at pH values 7–9. There is only one example of the type (iii) cluster (12) and this was synthesized from an existing108{V18} cluster [V18O42(SO4)]8by the addition of (NEt4)I in air.
It is important to note that the differences in the electron population of {V18O42} were identified and confirmed structurally using BVS, EPR, and magnetochemistry.106
Figure 14 A representation of the {V18þVO4} cluster (10), which includes a central {VO4} unit that could
be implicated as a template. The vanadium ions are shown as black spheres and the oxygen atoms as white spheres.
Table 2 Summary of the shell types and the formulas of the clusters characterized in each shell type.106
Shell type Compound formula VIV 18O42 Cs12[VIV18O42(H2O)]
14H2O (11a) K12[VIV18O42(H2O)]16H2O (11b) Rb12[V IV 18O42(H2O)]19H2O (11c) K9[H3V IV 18O42(H2O)]14H2O4N2H4(11d) K11[H2V IV 18O42(Cl)]13H2O2N2H4(13a) K9[H4VIV18O42(Br)]14H2O4N2H4(13b) K9[H4VIV18O42(I)]14H2O4N2H4(13c) K10[H3VIV18O42(Br)]13H2O0.5N2H4(13d) K9[H4VIV18O42(NO2)]14H2O4N2H4(13e) Cs11[H2VIV18O42(SH)]12H2O (13f) VIV16VV2O42 K10[HVIV16VV2O42(Cl)]16H2O (14a) Cs9[H2VIV16VV2O42(Br)]12H2O (14b) K10[HVIV16VV2O42(Br)]16H2O (14c) Cs9[H2VIV16VV2O42(I)]12H2O (14d) K10[HVIV16VV2O42(I)]16H2O (14e) K10[HVIV16VV2O42(HCOO)]15H2O (14f) Na6[H7VIV16VV2O42(VO4)]21H2O (10) VIV 10VV8O42 (NEt)5[VIV10VV8O42(I)] (12)This assembly principle can be extended to many types of anions. For example, the oxidation of [H9V19O50]8 in the presence of NEt4X appears to generate vanadium clusters of differing nuclearity dictated by the size of the anion, X. As an illustration of this, when X is azide a {V18} cluster results of the formula [H2V18O44(N3)]5 (15) seeFigure 16, for X is perchlorate a {V22} cluster is produced, [HV22O54(ClO4)]6(16) and when X is thiocyanate a {V22} cluster is also produced, [HV22O54(SCN)]6(17), seeFigure 17.
In an alternative synthetic procedure,97 the reaction of an aqueous solution of KVO3 with N2H5OH, followed by the addition of acetic acid to a pH of ca. 8 with heating yields crystals of [H2V22O54(OAc)]7(18). Changing the anion to NO3, by acidifying with HNO3instead of acetic acid, produces a {V18} cluster encapsulating a NO3(19) with the formula, [HV18O44(NO3)]10, see Figure 18. In another approach, Yamase et al. have used a photochemical method109 to synthesize some mixed valence {V18} clusters with a large number of vanadium(IV) ions, and they have also chosen azide as a template, [V18O44(N3)]14(20) and one example including phosphate, [V18O42(PO4)]11(21) which exhibits the same type of super-Keggin structure as observed for (10), [H7V
IV 16V
V
O42(VO4)]6.
The cluster [H2V18O44(N3)]5(15) has approximate D2hsymmetry and the cluster is built from edge- and corner-sharing tetragonal O¼VO4pyramids. The azide ion rests in the cavity with the shortest N
O distance being ca. 3.05 A˚ The {V22} clusters have very similar structures and are also comprised of tetragonal OVO4pyramids with an overall D2dsymmetry. In the case of the perchlorate cluster the perchlorate anion rests in the cavity with the shortest OO distance being around 2.96 A˚. However it appears that not only weakly bound anions can be incorporated into these cluster systems. It has also been possible to identify a [V34O82]10 cluster anion (22) that appears to incorporate a bonded {V4O4}O4cube within a cluster shell, seeFigure 19. The overall cluster anion [V34O82]10 has approximate D2d symmetry and consists of an ellipsoid-shaped {V30O74} sheath, which is formed by linking 30 tetragonal VO5 pyramids, and a central V4O4 cube. The sheath can be divided into two identical halves that are related by a 90 rotation withrespect to each other as defined by the geometry of the central cube. Geometrically each half of the anion contains 20 of the 24 oxygen atoms of a O24 rhombicuboctahedron. One interesting observation is that the {V18} sheath can be considered to be related to segments of a layer of vanadium pentoxide, seeFigure 20.64,65
Figure 15 A representation of the {V18} cluster, which encapsulates weakly coordinated small molecules at
its center (seeTable 1). The vanadium ions are shown as black spheres and the oxygen atoms as white spheres.
Figure 17 Structures of the {V22ClO4} (top left), (16),73,97{V22NCS} (disordered NCS) (top right), (17),97
{V22OAc}110(18) bottom center. The vanadium atoms are shown as black spheres and the oxygen atoms as
white spheres, and the other nonoxygen atoms of the anion are shown as gray spheres.
Figure 16 Structure of {V18N3} (20).73 The vanadium atoms are shown as black spheres and the oxygen
In summary it can be seen that linking VO5units in the presence of anions and other templates gives rise to several structural types that appear to be critically dependent on the nature of the anion, and this is summarized inFigure 21.
7.1.3 TUNGSTATES
The structural features of large polytungstate clusters can be visualized in terms of subunits based on lacunary fragments of the Keggin ion. In such representations many of the fragments may not actually exist independently from the cluster containing these subunits. This also applies to the building blocks identified as part of the polyoxometallate clusters discussed inSections 7.1.4and 7.1.5.
The Keggin ion (Figure 22) can adopt up to five skeletal isomers ("),114
and these isomers are related to each other by a rotation of one or more edge-shared {W3O13} groups by /3.56
Figure 18 The structure of the {V18NO3} cluster (19).111The vanadium atoms are shown as black spheres
and the oxygen atoms as white spheres, and the non-oxygen atoms of the anion are shown as gray spheres.
Figure 19 The structure of the {V34} cluster (22).112The vanadium atoms are shown as black spheres and
The relative stability of the different skeletal isomers of the Keggin ion, and indeed the equilibria that can exist between difference isomers, have been the subject of much discussion in the literature.115,116
Lacunary versions of these clusters geometrically result from the removal of one or more W atoms. Examples of one monovacant and two divacant lucunary derivatives of the -Keggin ion are shown in Figure 23.
The two tri-vacant species in Figure 23 correspond to the loss of a corner-shared group of {WO6} octahedral (A-type) or an edge-shared group (B-type). Furthermore it can be seen that in the B-type anion the central heteroatom has an unshared terminal oxygen atom; in this respect it is easy to imagine that B-type structures are observed when the central heteroatom has an unshared pair of electrons.
Figure 20 View of a segment of a layer of V2O5113with the terminal atoms omitted for clarity. The {V22}
framework is shown below. The vanadium atoms are shown as black spheres and the oxygen atoms as white spheres.
Figure 21 A schematic showing the outer vanadium–oxygen frameworks (vanadium ion shown as black spheres and oxygen atoms by white spheres) and the encapsulated molecules. The clusters depicted from; left to right (top) are the {V18-halide}, {V18-nitrate}, {V18-azide}, {V18-VO4}; left to right (bottom) are the
Linking of two [A-PW9O34]9clusters (23) generates a Wells–Dawson anion (24) [P2W18O62]6. Six isomers of this anion are theoretically possible depending upon whether the half-units are derived from or -Keggin species and also whether the fragments combined in a staggered (S) or eclipsed (E) fashion.114Four of these isomers have been observed for [As2W18O62]6and three for [P2W18O62]6.117As in the case of the Keggin ion, lacunary derivatives of the Wells–Dawson structure are also known. The most important of these are based on the most common isomer known as the -Dawson anion—seeFigure 24.
Figure 22 Polyhedral representation of the -Keggin ion. The metal ions form the centers of the polyhedra and the oxygen atoms form the apexes of the polyhedra. The central heteroatom is shown as a gray sphere.
{
α
-XW11} {A-α
-XW9} {B-α
-XW9}Figure 23 Polyhedral representation of the Lacunary derivatives of the -Keggin ion; left—{-XW11},
middle—{A--XW9}, right—{B--XW9}. The metal ions form the centers of the polyhedra and the oxygen
atoms form the apexes of the polyhedra. The central heteroatom is shown as a gray sphere.
α
-{X2W18}α
1-{X2W17}α
2-{X2W17} {X2W15}Figure 24 Polyhedral representations of the -Dawson structure the associated three lacunary derivatives. The metal ions form the centers of the polyhedra and the oxygen atoms form the apexes of the polyhedra.
7.1.3.1 Clusters Incorporating Monovacant Lacunary Fragments 7.1.3.1.1 {XW11}2
Monovacant lacunary anions (mla) of the type [PW11O39]7 (25), [SiW11O39]8 (26) and [P2W17O61]10(27) form both 1:1 and 1:2 complexes with several metal cations. Such complexes were first reported by Peacock and Weakley118in 1971. Recently 1:1 complexes of [-SiW
11O39] units have been shown to form polymers with lanthanide ions.119 It was suggested that the heteropolyanion ligands were tetradendate, binding to the lanthanide center through the four oxygen atoms that surround the tungsten vacancy. In this way a quasi-square antiprismatic coordination sphere is provided for the metal ion. An examination of the literature reveals many types of mla have been extremely well exploited as ligands with many combinations of metal ion (see reference84 for a further discussion of this point). Figure 25 gives a typical representation of one such [M(mla)2]ncomplex (28), which adopts a cis-oid conformation with respect to the mla ‘‘ligands.’’ This is because the polyanionic ligand only has Cs symmetry and therefore there are four possible conformations for the complexes corresponding to cis-oid and trans-oid enantiomeric pairs.120It would appear that all the Keggin-derived complexes are trans-oid whereas [M(2-P2W17O61)2](20n) (29) (M¼ CeIV, LuIII, UIV) are cis-oid. 1:1 and 2:2 com-plexes (2-P2W17O61) linked by lanthanide ions have also been recently reported.121
7.1.3.1.2 {XW11}2{Mo3S4}2
It has been shown by Mu¨ller et al.122 that the monovacant lacunary anion [SiW11O39]8 (30) reacts as an electrophile towards the nucleophile [Mo3S4(H2O)9]4þ(31) to yield a bridged species {(SiW11O39)2[Mo3S4(H2O)3]2(-OH)2}10(32). The resulting species has C2vsymmetry (seeFigure 26). The two nucleophilic lacunary anions, of the Keggin type {[SiW11O39]8} and of the Dawson type {[P2W17O61]10}, can be considered as negatively charged ligands which replace the coordin-ated water ligands of the electrophilic adduct. In this respect it has been suggested that the
Figure 25 Structure of [U(-GeW11O39)2]12(28). The ligands adopt a trans-oid conformation. A ball and
stick representation is shown on the LHS (black spheres¼ W atoms, white spheres ¼ O atoms and the visible gray sphere is the U atom). On the RHS the W atoms and the O atoms are represented as polyhedra.
